1. Field of the Invention
The present invention relates to a thermal-transfer recording sheet. More specifically, it relates to a recording sheet which is used in a thermal-transfer recording method in which an ink is transferred to the recording sheet by heating an ink layer of a thermal-transfer donor medium with a heating means, and which enables the high-quality recording of a signal, e.g., an image signal.
2. Prior Art
In recent years, a thermal-transfer recording method has been and is employed for facsimile machines, computer terminals, barcode recorders, printers and copying machines for the following reasons. It is noiseless due to the use of no impact and maintenance-free. It does not require a high cost, it can achieve a decrease in the size and weight of equipment, and it permits the recording in colors.
The thermal-transfer recording method refers to a method in which those portions of an ink layer on a thermal-transfer donor medium surface which correspond to recording signals are heated and melted to bring those portions of the ink layer into contact with a recording sheet and transfer those portions to the recording sheet. In the heat-melting ink layer used for this thermal-transfer recording, there are used inks prepared by dispersing a variety of pigments, aids, antistatic agents and fillers in wax-base binders such as paraffin wax, oxidized paraffin wax and carnauba wax or synthetic resin-base binders such as low-melting-point polyester, polyamide, a polyacrylic acid copolymer and a polystyrene copolymer.
In recording by the above thermal-transfer method, the recording sheet is required to receive ink layers of the above inks and permit the tight adherence of the inks. In general, a specially designed sheet is therefore used as the recording sheet.
It is now been studied to record data on a transparent recording sheet with a thermal-transfer type printer and use it as the data-recorded transparent sheet in an overhead projector (OHP). It is therefore increasingly desired to develop a sheet suitable for this purpose.
In terms of transparency, heat resistance and mechanical strength, plastic films such as a polyester film, a polyamide film, a polypropylene film and a polycarbonate film are used as the above transparent recording sheet. Since, however, these plastic films show poor adhesion to the above heat-melting inks, it cannot be said that these films can adequately receive the heat-melting ink layer.
When the adhesion between a plastic film and a heat-melting ink is inadequate, a so-called white spot occurs, which is a phenomenon that those portions of an ink layer in a molten state under heat which correspond to a signal are partially not transferred to a recording sheet. In particular, a white spot is liable to occur correspondingly to an edge portion of a recording signal and a narrow line portion thereof.
Color printers using a thermal-transfer method have been being widely used in recent years. When data is recorded in colors with a color printer, at most four kinds of heat-melting inks are to be transferred to one place of a recording sheet. When the recording sheet inadequately receives these inks, the print reproducibility is poor, and this phenomenon frequently occurs when the inks having a low color density are transferred. Further, when inks having a high color density are transferred, an intended final color is not reproduced, or there occurs a so-called white spot phenomenon that a specific color is, or specific colors are, not recorded.
The recent recording density by a thermal-transfer method is as high as 300 to 400 dots per inch (dpi), and when inks are thermally transferred in a low color density, the heat energy to be applied to each dot varies finely. And, each of ink layers of cyan, yellow, magenta and black are required to be transferred faithfully to a recording sheet correspondingly to fine variations of the heat energy.
Further, when the adhesion between a recording sheet and ink(s) is inadequate, the durability of an image on the recording sheet decreases. For example, sheets obtained by thermal transfer for an overhead projector are sometimes mutually brought into contact or printed surfaces of the sheets are rubbed against each other. When the durability of the ink layer is low, the ink layer peels off the sheet, and an image is badly impaired.
When recording sheets run through a printer, the following problems frequently occur. For example, there occur an overlapped feeding problem in which a plurality of sheets are fed out at one time from a tray, a jamming problem in which a sheet does not smoothly run due to a friction when it runs through rollers in the printer, and a feed-out problem in which a printed sheet sticks to another printed sheet on a tray to push it out or pull it into the printer when the former printed sheet is present on the tray. It is generally considered that these problems are mainly caused by electrostatic charge which has generated on the sheets.
Further, when a recording sheet is electrostatically charged, the printing surface of the recording sheet adsorbs dust around it to cause a white spot.
On the other hand, a printer is usually used in various environments. In particular, when a printer is used in a low-humidity environment, a recording sheet is liable to be electrostatically charged in printing, and the electrostatic charge causes the above white spot or a failure in ink transfer.
For the above reasons, it is strongly desired to carry out good antistatic treatment on recording sheet, and a variety of methods have been and are proposed.
Naturally, however, it should be avoided to impair the transparency of a recording sheet and the adhesion between a recording sheet and inks due to the antistatic treatment.